travis: fix compiler warnings for 1.22

src/comm/comm.rs

@@ -65,9 +65,9 @@ fn comm(a: &mut LineReader, b: &mut LineReader, opts: &getopts::Matches) {
 
     let delim : Vec<String> = (0 .. 4).map(|col| mkdelim(col, opts)).collect();
 
-    let mut ra = &mut String::new();
+    let ra = &mut String::new();
     let mut na = a.read_line(ra);
-    let mut rb = &mut String::new();
+    let rb = &mut String::new();
     let mut nb = b.read_line(rb);
 
     while na.is_ok() || nb.is_ok() {

src/printf/tokenize/unescaped_text.rs

@@ -179,7 +179,7 @@ impl UnescapedText {
         let mut new_text = UnescapedText::new();
         let mut tmp_str = String::new();
         {
-            let mut new_vec: &mut Vec<u8> = &mut (new_text.0);
+            let new_vec: &mut Vec<u8> = &mut (new_text.0);
             while let Some(ch) = it.next() {
                 if !addchar {
                     addchar = true;

src/unexpand/unexpand.rs

@@ -137,7 +137,7 @@ fn next_tabstop(tabstops: &[usize], col: usize) -> Option<usize> {
     }
 }
 
-fn write_tabs(mut output: &mut BufWriter<Stdout>, tabstops: &[usize],
+fn write_tabs(output: &mut BufWriter<Stdout>, tabstops: &[usize],
               mut scol: usize, col: usize, prevtab: bool, init: bool, amode: bool) {
     // This conditional establishes the following:
     // We never turn a single space before a non-blank into